1. Field of the Invention
The present invention relates to a silicon seed rod assembly used for producing polycrystalline silicon in a polycrystalline silicon reaction furnace, and a method of forming the same. Further, the present invention relates to a polycrystalline silicon producing apparatus employing the silicon seed rod assembly and a method of producing polycrystalline silicon.
Priority is claimed on Japanese Patent Application No. 2007-338409, filed Dec. 28, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
In general, the Siemens method is known as a method of producing high-purity polycrystalline silicon used as a semiconductor material. The Siemens method is a method of making raw gas composed of mixed gas of chlorosilane and hydrogen to contact with a heated silicon seed rod assembly and of making polycrystalline silicon deposited on a surface of the silicon seed rod assembly by means of a reaction of the raw gas. In an apparatus for adopting the Siemens method, a polycrystalline silicon reaction furnace is used in which a plurality of silicon seed rod assemblies is uprightly provided in a hermetic reaction furnace (see Japanese Patent No. 3819252 and Japanese Patent No. 3881647).
Each silicon seed rod assembly includes two rod-shaped silicon seed rods and a connection member connecting the upper end portions of the silicon seed rods to each other, thereby forming the silicon seed rod assembly in a Π-shape. Then, the lower end portion of the silicon seed rod is fixed to an electrode provided in a bottom of the polycrystalline silicon reaction furnace. The silicon seed rod assembly is heated by supplying current from the electrode at the lower end portion thereof. Then, the raw gas is thermally decomposed or is reduced with hydrogen by joule heat produced at this time, thereby making polycrystalline silicon deposited on the surface of the silicon seed rod assembly.
A section of the rod-shaped silicon seed rod is formed in a square shape, a circular shape, or a shape in which angular portions of a square shape are R-chamfered. In general, since it is necessary to make the upper end portion to be inserted through a through-hole having an annular-hole shape and to support the connection member, the upper end portion connected to the connection member is formed in a cylindrical shape so as to have a diameter slightly smaller than that of the through-hole.
That is, a step portion is formed between an upper end portion and a main body portion of the silicon seed rod since a width of the upper end portion is smaller than that of the main body portion. The step portion is provided with a flat support surface disposed in a direction perpendicular to a longitudinal direction of the silicon seed rod. Then, the upper end portions of the two silicon seed rods are inserted through the through-holes formed in both end portions of the connection member, and the support surface supports an opening-end peripheral edge of the through-hole on the side of the lower surface of the connection member, thereby forming the silicon seed rod assembly.
Incidentally, upon forming the through-hole in the connection member, the through-hole is formed by perforating the connection member by means of a drill from the one side surface toward the other side surface thereof. However, a petal-shape defect portion may be formed in the opening-end peripheral edge on the side of the other side surface in a perforation direction due to brittleness of the connection member. In a case where the silicon seed rod assembly is formed so that the other side surface having the defect portion formed thereon comes into contact with the support surface, a contact area between the connection member and the silicon seed rod decreases.
In a case where a contact state between the silicon seed rod and the connection member is poor, a problem arises in that a meltdown of the contact portion may occur due to local overheating during an ohmic heating. In order to obtain a satisfactory contact state between the silicon seed rod and the connection member, a study has been carried out which improves perpendicularity upon connecting the silicon seed rod and the connection member to each other in order to obtain a satisfactory contact between the silicon seed rod and the connection member. However, a countermeasure for coping with a contact between the support surface of the silicon seed rod and the opening-end peripheral edge of the through-hole of the connection member has not been taken. For this reason, in order to prevent the meltdown caused by overheating, it is required to satisfactorily ensure the contact area therebetween.